The invention pertains to a process for preparing the walls of a mold for the molding or shaping of a molded part after completion of a molding cycle and removal of the molded part from the mold to make the mold ready for the next molding cycle, comprising the following steps:
(a) the mold walls are brought to the desired temperature; and PA1 (b) a mold wall treatment agent is applied to the walls of the mold.
Processes of this type are known according to the state of the art and are used, for example, in the production of molded parts by molding processes such as those known in professional circles under names such as mold-casting, thixo-casting, thixo-forming, Vacural mold-casting, squeeze casting, etc. The state of the art will be explained below by way of example on the basis of the preparation of the mold walls of a mold for the die-casting of metal, but it is to be emphasized that analogous problems also occur in other shaping processes such as forging.
To produce a molded part, liquid or semi-liquid metal consisting of a light metal or heavy metal alloy is usually introduced under pressure into a divided, closed mold of steel and allowed to solidify. At the same time, the mold heats up as a result of the heat transferred to it from the solidifying material. Under production conditions, that is, during the production of as many castings as possible in the shortest possible time, the temperature of the mold would continue to increase. To achieve good-quality castings, however, the mold should have the same initial temperature at the start of each production cycle. Under production conditions, therefore, the mold must usually have heat removed from it continuously, so that thermal equilibrium is reached between the quantity of heat which the metal transfers to the mold and the quantity of heat which the mold releases as radiation to the surroundings or which is removed from it by supplemental cooling, with the result that an approximately uniform mold temperature is maintained.
Of course, instead of supplemental cooling, it may also be necessary to provide supplemental heating to the mold. This will be the case, for example, when only a small amount of metal is poured into a very heavy mold, that is, when molded parts with very thin members are produced. In this case, therefore, it can happen that the mold radiates off more heat to the surroundings that is desirable for the maintenance of a mold temperature favorable to the casting process. Therefore, with reference to the present invention, it is said in very general terms that the mold is "tempered", to cover both the possibility that the mold must be cooled as well as the possibility that it must be heated.
In addition to the need to temper the mold, it is also necessary to treat the surface of the mold walls with a lubricating and mold-release agent after removal of the last molded part and before the introduction of fresh liquid metal into the mold. This mold wall treatment agent has the primary job of preventing the introduced metal from welding or sticking to the material of the mold, of guaranteeing that the finished part can be removed from the mold, and of lubricating the moving parts of the mold such as the ejectors or pushers. In certain processes, the mold wall treatment agent has the additional task of reducing the heat transfer between the introduced metal and the mold during the filling process. The layer of mold wall treatment agent applied to the mold wall should have the most uniform possible thickness, because the layer can rupture at points where it is too thin, and this will result in turn in the welding of the introduced metal to the mold material. If the layers are too thin, furthermore, too much heat can be transferred from the introduced metal to the mold, with the result that the introduced metal cools down too quickly just after it has been introduced and thus prevents the mold from being filled sufficiently. But layers which are too thick can also impair the quality of the castings by occupying too much of the volume of the mold.
According to the conventional method, the mold walls are sprayed with a mixture of mold wall treatment agent and water each time a molded part is removed from the mold, as described in, for example, DE 4,420,679 A1 and DE 195-11,272 A1. The advantage of the use of these mixtures of treatment agent and water is the savings in time, which results from the fact that the surface of the mold wall is cooled by the sprayed-on water at the same time that the mold wall treatment agent is applied to the walls. One of the problems which has had to be dealt with in this method, however, is the Leidenfrost effect. That is, when the droplets of spray land on the hot surface of the mold wall, a vapor barrier forms between the droplets and the surface. This barrier prevents the droplets from completely wetting the surface. Some of the sprayed-on mixture of treatment agent and water therefore runs off the surface of the mold wall without cooling it, lubricating, it, or wetting it, and giving it the required release properties.
To cool and the mold wall surface and to be able to coat it with mold wall treatment agent sufficiently in spite of this problem, it is necessary to apply an excess of the treatment agent-water mixture. But then the trade-off must be accepted that a considerable amount of the treatment agent-water mixture will run off the surface of the mold walls unused and then must be collected and disposed of. This raises significant problems in terms of environmental compatibility, which will be explained in greater detail below on the basis of an example.
If we assume that a foundry uses approximately 5 kg of mold wall treatment agent concentrate per 1,000 kg of cast aluminum and that this concentrate is diluted with water in a ratio of 1:100 before spraying, i.e., a total of about 500 liters of treatment agent-water mixture is sprayed, and if we also assume that about 80% of this amount runs off unused from the mold walls as excess, this means that approximately 400 liters of waste liquid must be disposed of per ton of cast aluminum. This is a conservative estimate. A less favorable but equally realistic estimate results in a volume of approximately 900 liters for disposal per ton of aluminum. In a medium-sized casting shop with a capacity of about 5,000 tons of aluminum per year, it is therefore necessary to dispose of 2,000-4,500 m.sup.3 of waste liquid.